1. Field of the Art
The present invention relates to a piezoelectric ceramic material having the potential to find wide applications in fields including resonators and high-temperature pressure sensors.
2. Background Art
A piezoelectric material has the piezoelectric effect that causes an electric polarization change when it receives external stresses and the inverse piezoelectric effect that causes distortion at an applied electric field. Piezoelectric materials are now applied to sensors, resonators, actuators, etc.
Piezoelectric materials currently put to practical use, for the most part, are generally provided by ferroelectric materials having perovskite structures based on tetragonal or rhombohedral crystal system PZT (PbZrO3-PbTiO3 solid solutions) or tetragonal crystal system PT (PbTiO3). To meet a variety of requirements, a variety of subordinate components are added to these materials.
However, most of PZT or PT piezoelectric materials have Curie points of the order of 200 to 400xc2x0 C. on practical compositions, and have difficulty in high-temperature applications because they become normal dielectric materials and lose their piezoelectric properties at temperatures higher than those. These lead-base piezoelectric materials are also not preferable in view of ecological consideration and prevention of pollution, because of containing a large amount (about 60 to 70% by mass) of lead oxide (PbO) having extremely high vaporization even at low temperatures. For instance, when these lead-base piezoelectric materials are produced in the form of ceramics or single crystals, very large amountsxe2x80x94even on industrial levelsxe2x80x94of vaporous lead oxide are released by vaporization and diffusion into the atmosphere through unavoidable thermal treatments such as firing and melting. Lead oxide released during material production may be recovered; however, lead oxide contained in piezoelectric commodities on the market can hardly be done under the present situations. This lead oxide, when spewed into the environment over a wide area, would result inevitably in air pollution.
For piezoelectric materials that do not contain lead at all, for instance, BaTiO3 having a perovskite structure belonging to the tetragonal crystal system is well known. However, this material is less than practical because of having a Curie point as low as 120xc2x0 C.
For piezoelectric materials having a relatively high Curie point, for instance, a bismuth layer compound is known. The bismuth layer compound is represented by the following general formula:
(Bi2O2)2+(Amxe2x88x921BmO3m+1)2xe2x88x92
In this general formula, the element A is Ca, Sr, Ba or the like and the element B is Ti, Nb, Ta, W or the like. Referring to the bismuth layer compound, the c axis of the crystal is longer than the a and b axes and the more the value of m in the aforesaid general formula, the longer the c axis is. For compounds represented by the aforesaid general formula where m=2, for instance, SrBi2Nb2O9, BaBi2Nb2O9 and CaBi2Nb2O9 are known (A RESEARCH REPORT ON TRENDS IN PIEZOELECTRIC MATERIALS, Table 4, page 17 published by the Association of Electronic Material in March 1976). JP-A 11-322426 discloses a piezoelectric ceramic composition comprising SrBi2Nb2O9 and Mn added thereto in amount of 1.0% by weight or less as calculated on an MnCO3 basis, and states that the electromechanical coupling factor is improved by the addition of Mn. Until now, however, no prior art shows that BaBi2Nb2O9 and CaBi2Nb2O9 have piezoelectric properties.
Most of bismuth layer compounds, because of having high Curie points, have properties satisfactory enough for high-temperature sensors. However, a problem with some bismuth layer compounds free from lead whatsoever is that their Qmax important for resonators is small. Here Qmax is defined by tan xcex8max where xcex8max is the maximum value of a phase angle. That is, Qmax is the maximum value of Q (=|X|/R) between a resonant frequency and an anti-resonant frequency, where X is reactance and R is resistance. The larger Qmax or the closer xcex8max is to 90xc2x0, the more stable oscillation is and the lower is the temperature at which oscillation occurs.
An object of the present invention is to provide a piezoelectric ceramic that is free from lead, has a high enough Curie point and possesses improved piezoelectric properties.
This object is achievable by the following embodiments (1) to (3) of the present invention.
(1) A piezoelectric ceramic material comprising a Bi3TiNbO9 crystal and/or an MBi2Nb2O9 crystal which are each a bismuth layer compound, where M represents at least one element selected from Sr, Ba and Ca, and containing Bi, Ti, M and Nb as main component elements, wherein a molar ratio as oxides of said main component elements is given by (Bi3xe2x88x92xMx)z(Nb1+yTi1xe2x88x92y)O9 provided that 0  less than x, y xe2x89xa60.8 and 0.95 xe2x89xa6z xe2x89xa61.05, and 0xe2x89xa6xBxe2x89xa60.5 and 0xe2x89xa6xC less than 0.4 wherein a molar ratio of Ba/(M+Bi) is given by xB/3 and a molar ratio of Ca/(M+Bi) is given by xC/3.
(2) The piezoelectric ceramic material of (1) above, wherein xxe2x88x920.5xe2x89xa6yxe2x89xa6x+0.5.
(3) The piezoelectric ceramic material of (1) or (2) above, which further contains as a subordinate component element at least one element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ta, Zr, Cr, Fe and W.
According to the present invention, it is possible to achieve a piezoelectric ceramic material that is free from lead and has a high-enough Curie point and a large value for the aforesaid Qmax.
The key composition for the piezoelectric ceramic material of the invention is a solid solution of Bi3TiNbO9 and MBi2Nb2O9. If the mixing ratio of MBi2Nb2O9 in this solid solution is limited to within the range defined by the invention, then the value of Qmax can be critically increased.
As already mentioned, SrBi2Nb2O9 is a known piezoelectric material. However, the Qmax of SrBi2Nb2O9 is small, as can be appreciated from the comparative samples in the examples of the invention. As again mentioned above, no prior art shows that BaBi2Nb2O9 and CaBi2Nb2O9 have piezoelectric properties. Also, no prior art shows that Bi3TiNbO9 has piezoelectric properties. As can be appreciated from the comparative samples in the examples of the invention, the Qmax of Bi3TiNbO9 is nearly close to zero. With the invention, it is possible to achieve an unexpected effect that some considerable large value is obtained for Qmax by mixing SrBi2Nb2O9 with limited Qmax and BaBi2Nb2O9 or CaBi2Nb2O9 with nothing reported about their piezoelectric properties in Bi3TiNbO9 with nothing reported about its piezoelectric properties.